Mosquito season in Merrimack County, NH: when it starts and ends
Mosquito months in Merrimack County
In a normal year mosquito season in Merrimack County, NH starts around May 3 and ends around October 9, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Merrimack County, and when
- Start tipping out standing water weekly by April 19.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 9.
- 1 West Nile case since 2016 (last 2023): you should use repellent at dusk July, more so if over 60.
Merrimack County against the rest of NH
Merrimack County has the 6th longest season of 10 NH counties, at 159 days against 170 in Strafford County and 141 in Coos County. Its mean temperature is 45.7°F.
- Hillsborough County: April 30 to October 11 (164 days, 0 WNV)
- Cheshire County: May 2 to October 10 (161 days, 1 WNV)
- Sullivan County: May 5 to October 7 (156 days, 0 WNV)
- Carroll County: May 7 to October 3 (150 days, 0 WNV)
West Nile in Merrimack County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Merrimack County (1 neuroinvasive) across 1 year. The worst was 2023 with 1; the latest was 2023.
More: how the dates are worked out · when the season starts across the US.